Put the pumpkin in a saucepan and add enough water to cover.
Add 1 tsp of salt and bring to a boil, lower the heat to a simmer for 20 minutes.
Drain and reserve the cooking liquid.
Melt the butter in a clean saucepan over medium heat.
Add the onion and green onions and fry them, stirring constantly until they are soft and golden, about 5 minutes.
Add the pumpkin, tomatoes, coconut milk, 3 cups of the cooking liquid, half the nutmeg, a pinch of cayenne pepper, salt and freshly ground pepper Bring to a boil, then lower the heat and simmer for 30 minutes.
Remove the pan from heat and leave it to cool slightly.
Puree half the soup at a time in a blender, then return to the pan.
Heat it through for 5 minutes.
Pour into warmed soup bowls and swirl a bit of the sour cream or yogurt in it and sprinkle fresh nutmeg on top Serve at once.